Physics F3 (IGCSE) speed, velocity and acceleration

PHYSICS – Speed, velocity and acceleration

LEARNING OBJECTIVES1.2 Motion Core • Define speed and calculate average speed from total time / total distance • Plot and interpret a speed-time graph or a distance- time graph • Recognise from the shape of a speed-time graph when a body is – at rest – moving with constant speed – moving with changing speed • Calculate the area under a speed-time graph to work out the distance travelled for motion with constant acceleration • Demonstrate understanding that acceleration and deceleration are related to changing speed including qualitative analysis of the gradient of a speed-time graph• State that the acceleration of free fall

for a body near to the Earth is constant

Supplement • Distinguish between speed and velocity • Define and calculate acceleration using time taken change of velocity • Calculate speed from the gradient of a distance-time graph • Calculate acceleration from the gradient of a speed-time graph • Recognise linear motion for which the acceleration is constant • Recognise motion for which the acceleration is not constant • Understand deceleration as a negative acceleration• Describe qualitatively the motion of bodies falling in a uniform gravitational field with and without air resistance (including reference to terminal velocity)

A

Average speed+= Distance moved Time taken

A

Average speed+= Distance moved Time taken

Distance measured in metres (m)Time measured in seconds (s)

Speed - metres per second (m/s)

A

Average speed+= Distance moved Time taken

Example:

Car travels 50m time 2s

speed = 50/2 = 25 m/s 25 m.s-1

So if that’s speed, what is

velocity?

Velocity is speed in a given direction.

Velocity is speed in a given direction.

Velocity is 25m/s due west

Example:

Example:

Example:

Example:

Cyclist +10m/s to the right

Example:

Cyclist +10m/s to the right-10m/s to the left

What’s your vector Victor?

What’s your vector Victor?

Quantities such as velocity are called

vectors because they have size and

direction

Acceleration is the rate at which an object increases speed or velocity.

Acceleration is the rate at which an object increases speed or velocity.Acceleration = change in velocity time taken

Acceleration is the rate at which an object increases speed or velocity.Acceleration = change in velocity time taken

Also written as: a = v - u t

Acceleration is the rate at which an object increases speed or velocity.Acceleration = change in velocity time taken

Velocity measured in m/sTime measured in s

Acceleration measured in m/s/s or m/s2

Example: a drag car increases its velocity from zero to 60m/s in 3s.

a = v - u t

Example: a drag car increases its velocity from zero to 60m/s in 3s.

a = v - u t

a = 60 – 0 3

Example: a drag car increases its velocity from zero to 60m/s in 3s.

a = v - u t

a = 60 – 0 3

a = 60 = 20m/s-2 3

Example: a drag car increases its velocity from zero to 60m/s in 3s.

a = v - u t

a = 60 – 0 3

a = 60 = 20m/s-2 3

Don’t forget that acceleration is a vector

– it has size and direction

Deceleration (retardation)

Deceleration is negative

acceleration – the object is slowing

down.Eg. – 4m/s2

Constant acceleration example

A B6s

Car passes point A with a velocity of 10m/s. It has a steady (constant) acceleration of 4m/s2. What is the velocity when it passes point B?

Constant acceleration example

A B6s

Car passes point A with a velocity of 10m/s. It has a steady (constant) acceleration of 4m/s2. What is the velocity when it passes point B?

Solution: car gains 4m/s of velocity every second. In 6s it gains an extra 24m/s.

Constant acceleration example

A B6s

Car passes point A with a velocity of 10m/s. It has a steady (constant) acceleration of 4m/s2. What is the velocity when it passes point B?

Solution: car gains 4m/s of velocity every second. In 6s it gains an extra 24m/s.

Final velocity = initial velocity + extra velocity

Constant acceleration example

A B6s

Car passes point A with a velocity of 10m/s. It has a steady (constant) acceleration of 4m/s2. What is the velocity when it passes point B?

Solution: car gains 4m/s of velocity every second. In 6s it gains an extra 24m/s.

Final velocity = initial velocity + extra velocity

Final velocity = 10 + 24 = 34m/s

Motion graphs

Travelling at constant speed

Travelling at constant speed

Stationary

Travelling at constant speed

Stationary

Travelling at constant speed

Speed = distance time

Speed = distance time

Speed = distance time

Speed = distance time

Speed = 8 = 1 km/h 8

Acceleration from velocity : time graph

Acceleration from velocity : time graph

Steady acceleration

Acceleration from velocity : time graph

Steady acceleration

Steady velocity

Acceleration from velocity : time graph

Steady acceleration

Steady velocity

Steady deceleration

Acceleration from velocity : time graph

Acceleration = V - U t

Acceleration from velocity : time graph

Acceleration = V - U t

Acceleration from velocity : time graph

Acceleration = 3 – 0 / 2 = 1.5 m/s/s (m.s-2)

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

Acceleration can be calculated by the gradient of a velocity:time graph. (Remember gradient is the difference up divided by the difference across)

Calculate the acceleration for each of the 4 sections of the graph.

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

Acceleration can be calculated by the gradient of a velocity:time graph. (Remember gradient is the difference up divided by the difference across)

Calculate the acceleration for each of the 4 sections of the graph.

Acceleration = V - U t

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

Acceleration can be calculated by the gradient of a velocity:time graph. (Remember gradient is the difference up divided by the difference across)

Calculate the acceleration for each of the 4 sections of the graph.

Acceleration = 40 - 0 = 4m/s2

10

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

Acceleration can be calculated by the gradient of a velocity:time graph. (Remember gradient is the difference up divided by the difference across)

Calculate the acceleration for each of the 4 sections of the graph.

Acceleration = 0 (no change in velocity)

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

Acceleration can be calculated by the gradient of a velocity:time graph. (Remember gradient is the difference up divided by the difference across)

Calculate the acceleration for each of the 4 sections of the graph.

Acceleration = 20 - 0 = 2m/s2

10

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

Acceleration can be calculated by the gradient of a velocity:time graph. (Remember gradient is the difference up divided by the difference across)

Calculate the acceleration for each of the 4 sections of the graph.

Acceleration = 0 - 60 = -3m/s2

20

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Remember that the area of a triangle is ½ x base x height.

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Remember that the area of a triangle is ½ x base x height.

Area = 200m2

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Remember that the area of a triangle is ½ x base x height.

Area = 200m2

Area = 400m2

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Remember that the area of a triangle is ½ x base x height.

Area = 200m2

Area = 400m2

Area = 400m2

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Remember that the area of a triangle is ½ x base x height.

Area = 200m2

Area = 400m2

Area = 400m2

Area = 100m2

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Remember that the area of a triangle is ½ x base x height.

Area = 200m2

Area = 400m2

Area = 400m2

Area = 100m2

Area = 600m2

Velocity-time graphs

80

60

40

20

010 20 30 40 50

Velocitym/s

Time/s

On a velocity – time (or speed – time) graph, the area under the line is numerically equal to the distance travelled.

Remember that the area of a triangle is ½ x base x height.

Area = 200m2

Area = 400m2

Area = 400m2

Area = 100m2

Area = 600m2

The total distance travelled = 200 + 400 + 400 + 100 + 600 = 1700m

Free fall

Acceleration of free fall (g)

Which object will hit the

ground first?

Acceleration of free fall (g)

Which object will hit the

ground first?

Obviously the brick (because the feather is slowed much

more by the air)

Acceleration of free fall (g)No air

resistance, objects both fall with the

same downward

acceleration.

In air In a vacuu

m

Acceleration of free fall (g)No air

resistance, objects both fall with the

same downward

acceleration.

In air In a vacuu

m

Acceleration of free fall = 9.8m/s2

Given the symbol ‘g’

Acceleration of free fall (g)No air

resistance, objects both fall with the

same downward

acceleration.

In air In a vacuu

m

Acceleration of free fall = 9.8m/s2

Given the symbol ‘g’

Often rounded to

10m/s2

Acceleration and gravity

Acceleration and gravity

Falling objects accelerate

towards the ground at 10m/s2 due to gravity.

The force of gravity always

acts towards the centre of the

Earth.

The atmosphere creates an upward

force that slows down falling

objects. This is known as air resistance or

drag.

Acceleration and gravity

Falling objects accelerate

towards the ground at 10m/s2 due to gravity.

The force of gravity always

acts towards the centre of the

Earth.

The atmosphere creates an upward

force that slows down falling

objects. This is known as air resistance or

drag.

Acceleration and gravity

Falling objects accelerate

towards the ground at 10m/s2 due to gravity.

The force of gravity always

acts towards the centre of the

Earth.

The atmosphere creates an upward

force that slows down falling

objects. This is known as air resistance or

drag.

The larger the surface area of the object, the larger the drag force

Acceleration and gravity

Spee

d (m

/s)

Time (s)

Drag

Weight

AAt first the force of

gravity is larger than the drag force, so the object

accelerates.

Acceleration and gravity

Spee

d (m

/s)

Time (s)

Drag

Weight

B As speed increases, so does drag; the

acceleration decreases

Acceleration and gravity

Spee

d (m

/s)

Time (s)

Drag

Weight

C

When drag equals the force due to gravity there is no resultant force and the acceleration is zero. The object continues at

terminal velocity.

Terminal velocity

LEARNING OBJECTIVES1.2 Motion Core • Define speed and calculate average speed from total time / total distance • Plot and interpret a speed-time graph or a distance- time graph • Recognise from the shape of a speed-time graph when a body is – at rest – moving with constant speed – moving with changing speed • Calculate the area under a speed-time graph to work out the distance travelled for motion with constant acceleration • Demonstrate understanding that acceleration and deceleration are related to changing speed including qualitative analysis of the gradient of a speed-time graph• State that the acceleration of free fall

for a body near to the Earth is constant

Supplement • Distinguish between speed and velocity • Define and calculate acceleration using time taken change of velocity • Calculate speed from the gradient of a distance-time graph • Calculate acceleration from the gradient of a speed-time graph • Recognise linear motion for which the acceleration is constant • Recognise motion for which the acceleration is not constant • Understand deceleration as a negative acceleration• Describe qualitatively the motion of bodies falling in a uniform gravitational field with and without air resistance (including reference to terminal velocity)

PHYSICS – Speed, velocity and acceleration